It is highly desirable to feed most molten metals into any casting arrangement in such a manner that there is minimal contact with an uncontrolled atmosphere. To accomplish this protection of the molten metal from an uncontrolled atmosphere in twin-belt continuous casting, the caster is set up for "closed feeding," a term which includes both closed-pool feeding and injection feeding. Specific features of these latter two techniques are not germane here but are explained in U.S. Pat. Nos. 4,593,742 and 4,648,438, both of which are assigned to the same assignee as the present application. The synonymous terms "closed feeding" or "closed metal feeding" or "closed casting" do not mean entire sealing (air-tight sealing) of the upstream or feeding end of the moving mold cavity defined between the two moving belts, but rather such terms mean substantially blocking the entrance of the moving mold cavity by a metal-feeding nozzle with respective clearances around the nozzle in a range roughly up to about 0.050 of an inch (about 1.27 mm). Usually the clearances around the nozzle are less than that figure, as discussed in the referenced patents. Closed metal feeding is always used for twin-belt casting of aluminum, and it is also used where feasible in such continuous casting of slab of any metal having a melting point higher than that of zinc.
In the continuous casting of, say, aluminum between endless flexible metallic casting belts, a metal-pouring nozzle comprising multiple channels of closed cross-section is generally used to conduct the molten aluminum into the twin-belt casting machine. Such a nozzle having channels (feeding passageways) of closed cross-section protects the molten metal from oxidation and undue heat loss, which would be caused by contact with ambient air and which otherwise would occur, if open runners were used. To protect the molten metal from ambient air, the prior art has used closed conduits made of refractory materials, often ceramics, the walls of which have not been permeable to gas. It has generally been assumed heretofore that such gas-nonpermeability was very desirable in the twin-belt continuous casting of molten metals, since oxidation of molten metals is a common problem in casting operations.
This gas-impermeability of the molten-metal-feeding nozzle is especially advantageous, for instance, when casting molten steel, where uncontrolled atmospheric contact results in the formation of unwanted oxides and nitrides. The steel industry has taken pains to develop impervious conduit materials. The impermeability of prior-art nozzle materials has been turned to further advantage by conducting inert shielding gases directly into the casting area through long holes drilled in nozzles made of such impervious materials, as taught in U.S. Pat. Nos. 4,593,742 and 4,648,438 relating to inert gas shrouding apparatus and methods.
For the purpose of excluding atmospheric gases, the prior art known to me for closed metal feeding of twin-belt continuous casting machines has always incorporated metal-feeding snouts or nozzles that were practically impermeable to gas. A typical material for the refractory nozzles in the prior art of twin-belt continuous casting of aluminum has been a baked clay that contains asbestos or, more recently, compressed and mildly baked calcium silicate. Although such impermeable refractory nozzles enabled the twin-belt casting of aluminum to develop to a high state of usefulness, some problems remained.
Such dense, non-permeable refractories have always suffered from certain drawbacks, notably brittleness and inflexibility. For example, prior experiments were made with non-permeable ceramics. Fired ceramic nozzles have been apt to crack when a minutely warped nozzle was clamped into position for casting. Surface grinding of the broad faces of the ceramic nozzles was tried in order to get rid of the warp caused by firing, but micro-cracks would develop upon grinding, resulting in reduced strength and reduced thermal shock resistance under the conditions of service. Even without a detectable initial warp, cracking was apt to occur in non-permeable ceramic nozzles due to uneven thermal expansion and the consequent tendency to warp. Interrupting with shallow grooves the outer broad surfaces of ceramic or other solid earthenware nozzles reduced troublesome thermal stresses and cracking of nozzles but has not been a sufficient solution to the problems which remained.
A very troubling problem with nozzles made from non-permeable materials and used for feeding molten aluminum into twin-belt casters has been the mysterious occurrence of gross voids in the continuously cast metallic slab product.
It became my theory that entrained gas caused these gross void spaces, which measured on the order of 1/4 inch (6 millimeters) in diameter. Subsequent rolling of such cast slab containing such voids would result in corresponding perforations appearing in the rolled, thin, sheet strip.
I had the theory first that the gas causing such voids came from the nozzle material itself, and some of it did. Some nozzle materials contained carbonate or hydrate that would break down at high temperature and evolve gas. Such gas evidently became entrained in the flow of molten metal, coalescing into large bubbles, and so moved downstream in the freezing product, where the voids were later found. Usually they were just under the upper surface of the cast slab, sealed usually with a thin film of aluminum that was level with the top surface of the cast slab.
In order to get rid of the entrained gas, thorough preheating and consequent outgassing of impermeable refractory nozzles was tried. Such outgassing of non-permeable nozzles prior to their use improved the situation. However, intense prior baking and outgassing of such nozzles, even in a vacuum, consistently failed to stop the formation of mysterious gross voids, despite tests with many formulations and grades of non-permeable nozzle material.
The mysterious voids kept on appearing, as just described. I then began to suspect that some other source was introducing gas into the molten metal flowing downstream into the moving mold region between the two moving belts.
It is known in aluminum metallurgy that molten aluminum and its alloys often contain dissolved hydrogen and moreover the surprising fact that the solubility of this hydrogen in the aluminum decreases with decreasing temperature. It became relevant to my theory to note that in twin-belt continuous casting of aluminum, the nozzle does not receive external heat. Thus, I reasoned that the temperature of the molten aluminum must have decreased as it traversed the passages of the non-permeable nozzle. I carefully observed and repeatedly noted that the inner surfaces of the aforementioned sealed gross voids in the cast aluminum product were always shiny. If the troublesome gases had contained oxygen, as the earth's atmosphere does, I would expect such inner void surfaces to be noticeably oxidized to a dull, non-shiny appearance. Such a non-shiny appearance was not the case; consequently I concluded that the offending gases did not contain much oxygen.
Since outgassing of non-permeable nozzles even in a vacuum did not solve the problem of the mysterious gross voids, and since my careful repeated inspection of the walls of such voids revealed them to be shiny, it became my theory that the above facts pointed to another non-atmospheric source of the remaining gas in the gross voids. I suspected hydrogen to be the offending gas, coming from the molten aluminum itself. In this theory, I concluded that the offending gas was expelled from solution during travel of the molten aluminum through the nozzle, rather than later. In addition to the temperature drop occurring in the nozzle, it may be that turbulence (such as I believe to exist in the nozzle passages) contributes to the separation (liberation) of the dissolved gas from the molten aluminum.
In summary, the facts suggested to me a theory that the offending gas is hydrogen and that it is released from the molten aluminum while it is flowing through the nozzle, such release of hydrogen possibly being augmented by turbulent flow through the nozzle passages.
In order to test wider applicability of my theory, an associate poured molten copper through non-permeable quartz (fused silica) tubes. Voids appeared on the upper surface of the cast product in the form of black streaks of bubbles. This lone experiment suggests that aluminum is not the only metal incurring the problem here considered.